1. Technical Field
The present invention relates to electronic compasses and more specifically to a method for selecting calibration data for an auto-calibrating electronic compass.
2. Discussion
Normally, electronic compass systems employ a microprocessor to calculate vehicle headings based upon signals received from a magnetic flux-gate sensor. An example of a microprocessor-based electronic compass can be found in U.S. Pat. No. 4,622,843 to Hormel, issued Nov. 18, 1986, entitled "Simplified Calibration Technique and Auto-Ranging Circuit for an Electronic Compass Control Circuit". This reference is hereby incorporated by reference.
The operation of the flux-gate sensors are also well-documented. See for example, "Magnetic Field Sensor and its Application to Automobiles" by Hisatsugu Itoh, dated February, 1980 and published by the Society of Automotive Engineers as Paper No. 800123; "Magnetic Heading Reference for the Electro/Fluidic Autopilot", by Doug Garner which appeared in two parts, one in the November, 1981 and one in the December, 1981 issues of Sport Aviation. The above-mentioned publications are also hereby incorporated by reference.
The subject invention is closely related to and is an improvement upon the method described in commonly assigned U.S. Pat. No. 4,807,462 to Al-Attar issued Feb. 28, 1989 entitled "Method for Performing Automatic Calibration in an Electronic Compass". This document is hereby incorporated by reference.
Auto-calibrating compasses suffer from real world problems. As discussed in the '462 patent to Al-Attar, auto-calibration occurs as a vehicle equipped with an auto-calibrating compass is driven. Since the calibration process does not occur at the same spot on the earth, it is subject to the effects of minor variations in the earth's magnetic field. Magnetic anomalies, caused by locally high concentrations of ferrous minerals, and magnetic transients, caused by time-variant magnetic fields, create noise in flux-gate sensor readings. It is desirable that the auto-calibration method minimize the effects of the noise.
A method for calculating the vector for the noise caused by magnetic anomalies and transients is disclosed in German Patent No. DE 3644681 Al, issued Jul. 14, 1988, to Helldorfer. A microprocessor is used to calculate the difference between a magnetic field vector sensed by the compass sensor and a previous magnetic field vector for the same heading. The resultant vector is then weighted by a factor k&lt;1, which suppresses the effect of the anomalies and transients upon the direction of the vehicle.